Either follow this excellent guide for CLI:
Or download and follow this for the GUI tool:
First fill out the fields:
To add your .dll, simply build your project in Release mode, locate the .dll and drag it over the tool.
Then, when all metadata is entered:
You can get an API key by registering at http://nuget.org/ and then enter it to the 'Publish key' field.
After a successful upload the package would be available from Package Manager Console:
PM> Install-Package Base64